This invention relates to positive drive power transmission systems composed of toothed belts in combination with toothed pulleys and methods for making such toothed belts. More particularly, the invention is directed to positive drive power transmission systems in which the operating noise resulting from the impact of the toothed belts against their corresponding toothed pulleys is reduced substantially.
U.S. Pat. No. 2,507,852, issued on May 16, 1952 to R. Y. Case, describes in detail a power transmission belt comprising an inextensible tensile member having teeth bonded to it on one side. A protective jacket fabric covers the teeth. The teeth are made of an elastomeric material, such as rubber. The belt is designed to mesh with one or more toothed pulleys that are constructed of material having a higher Young's modulus than the elastomeric material of the belt teeth.
Such toothed belts have been constructed with teeth of various cross-sectional configurations. The tooth cross-sectional configuration shown in the Case patent is trapezoidal, similar to that of a conventional rack gear tooth. U.S. Pat. No. 3,756,091, issued on Sept. 4, 1973 to H. F. Miller, describes in detail a similar power transmission belt where the tooth cross-sectional configuration is substantially curvilinear being partially composed of two intersecting arcs. Different tooth cross-sectional configurations have been described in patents such as U.S. Pat. No. 3,924,481, issued on Dec. 9, 1975 to M. J. W. Gregg; U.S. Pat. No. 4,037,485, issued July 26, 1977 to R. D. Hoback; U.S. Pat. No. 4,066,732, issued Jan. 3, 1978 to J. D. Redmond; and U.S. Pat. No. 4,108,011, issued Aug. 22, 1978 to M. J. W. Gregg et. al., and in the literature. Toothed power transmission belts having cross-sectional configurations as described in the patents to Case and Miller, as well as belts having teeth of other configurations.
In any such positive drive power transmission system, the belt and pulleys forcefully engage or collide with each other when they mesh. A noise results from this impact contact.
The characteristic sound of such a positive drive power transmission system in operation is a loud "whine". The whine is comprised primarily of the meshing frequency of the system i.e. the frequency of collisions or impact contacts between the toothed belt and its pulleys and the integer harmonics of the meshing frequency. The meshing frequency is calculated by multiplying the number of grooves in the pulley by the pulley's speed of rotation. For example, a belt running on a 30-groove pulley turning at 40 revolutions per second would generate a noise composed of a 1200 Hz fundamental frequency and its integer harmonics, i.e. 2400 Hz, 3600 Hz, etc. When positive drive power transmission systems are operated under conditions of high horsepower, torque, and speed, noise levels in excess of 80 dB normally result. It is not uncommon for noise to be generated in excess of 90 dB.
Investigations of the effects of noise on hearing have shown that exposure to sound levels equal or greater than 85 dB for lengthy periods of time may result in permanent hearing loss. Thus various government regulations have been promulgated controlling exposure to high noise levels. As a result of such regulation and as a safety precaution, it is becoming common in industry to limit the durations of time a worker can be exposed to high noise levels and to require the wearing of ear protective devices.
Limitations related to noise have prevented the widespread use of toothed belt drives in certain industries. For example, the textile industry utilizes yarn twisting machines which are driven by V-belts. The energy required to operate them could be reduced substantially if toothed belts could be substituted for V-belts. However, the drive conditions and background noise levels are such that added noide due to positive drive power transmission systems could not be tolerated.
Until the present invention there was no way to modify the belt or pulleys in a positive drive power transmission system to significantly reduce the noise levels in those cases where the impact contact between the belt and pulleys produces unacceptably high noise levels. Noise could only be reduced by surrounding the entire positive drive power transmissions system with an acoustic guard or enclosure. The use of an acoustic guard or enclosure is not only costly, but adds to the problems of maintenance, and trapped heat within the enclosure, and to the bulk of the machine.